Abrasive article and method of making the same



Oct. 8, 1940. R. c. BENNER Er Al. 2,216,728

ABRASIVE ARTICLE AND METHOD OFMAKING THE SAME Original Filed Deo. 31,1935 PREBONDED AGGREGATES A a s fr( N lNTERAGC-:REGATE BOND INVENTORS.

RAYMOND C. BENNER.

ATTORNEY.

Patented Oct. 8,

UNITED. STATES PATENT Acarica ABRASIVE ARTICLE AND LIETHOD F MAK- INGTHE SAME Application December 31, masa-1an No. 57,060 Renewed June 30,1939 4 Claims.

y This invention relates to the production of bonded abrasive articleswherein the abrasive granules are held firmly in a setting of .binderwhereby they continue to cut until worn away.

5 This is in contradistinction to the action of ordinary abrasivearticles wherein it is intended that when an abrasive granule is worn todullness the increased friction causes the granule to fracture, exposinga fresh cutting edge, or to break out of the bond structure entirely,making way for bringing into action an unworn granule. In furthercontrast to ordinary abrasive articles, those with which the presentinvention is concerned are extremely dense with vsubstantially all thespace between the abrasive granules lled with binder and requireperiodical sharpening to bring y fresh cutting edges into play, whereasordinary abrasive articles are somewhat porous to facilitate -the actiondescribed above.

An extremely dense structure is made necessary by the character ofthework to ybe done, or by the nature of the abrasive material used. Incutting certain extremely hard materials arelatively porous bondstructure would be so weak as to let loose the abrasive granules at theviirst attempt to cut the material thereby making the cutting operationineflicient with respect to the work. 4In the use of certain ratherexpensive abrasives, such as diamonds and boron carbide, this prematurebreaking out of the granules before they have been completely used upwould render the process ineiiicient with respect to the abrasive,regardless of the work.

It has been suggested to bond abrasive granules into dense cutting tools.by means of metals, for example by mixing them with molten metal suchas zinc or by mixing the abrasive specifically diamonds, with finelydivided electrolytic iron particles and molding the mixture under ogreat pressure to form a dense homogeneous article.

Such articles have a number of disadvantages however. As stated abovethey must be dressed or sharpened periodically as the granules wear toremove Ibond from around the granules to enable them to penetrate thework, and provide space for accumulation of material cut from the work.This is dimcult to do without unduly loosening the granules from theirfoundation, especially in the case of finely divided abrasive materialbecause of the small spaces between adjacent grains and the relativesoftness of the bonds used. For this reason it is practically impossibleto use satisfactorily in this way extremely fine abrasive ma- Il terial,such as the diamond dust produced in crushing and otherwise preparinglarger granules for use. Furthermore there are certain disadvantages inthe use of dense metal .bonds with respect to the action of the wheel asa whole. More accurately it might be saidA that there are 5 certainvaluable characteristics of an abrasive article bonded with othermaterials, such as resins,

or rubber which are not present in a metal article. 'I'hese advantagesare dimcult to deiine but no less present and may be said to give agrind- [lo ing wheel better grinding characteristics.

It is an object of our invention therefore to provide a method of makingabrasive articles, of the dense type described above, by which thecutting properties of the article can .be controlled i5 closely within awide range of variation; to provide a method wherein a binder forholding the abrasive grains in their setting and a binder for providingthe wheel structure are selected as hav--` ing the properties bestsuited to the purpose lfor zo which they are intended; to provide amethod wherein abrasive grains are formedv into aggregates by means of abinder having the properties of thoroughly wetting the grains whenliquid or plastic and forming a rela-tively hard brittle bindg5 er forthe grains when set, the aggregates being then bonded into a dense bodyby a binder which is liquid or plastic at a temperature which does notinjure the rst binder whereby it flows, with or without pressure, intoall spaces between the oy aggregates and all spaces communicatingtherewith within the aggregates. It is a further object to provide amethod of making abrasive articles wherein the cutting qualities of thefinished article can be selectively determined by varying u theproperties of the binders used with respect to the grain used and withrespect to each other. It is a further object to provide a structurewhich combines the advantages of the use of small abrasive granules withthe cutting power and clearance spaces characteristic of abrasivestructures utilizing large abrasive granules. It is also an object ofthe invention to provide a means ofv utilizing abrasive dusts whichcould otherwise not be utilized in a bonded condition, in the producation of abrasive articles whose properties approach 'those of articlesformed of larger abrasive granules.

The aggregates may be formed in a manner to make them somewhat porouswhereby the binder 5g used to form the abrasive article penetrates thepores of the aggregates to assist in holding the aggregates in place, orthe aggregates maybemade to have a dense structure with substantially novoids therein. 55

The binder used to form the aggregates may be any one suitable for usein binding abrasive grains including those already used in theproduction of abrasive articles. These include inor- 5 ganic binderssuch as metals, clays and glasses, and organic binders such as resins(thermoplastic and heat hardenable) rubber, and cellulose derivatives.'I'he method of making the aggregates will depend on the nature of thebinder used and will l0 be described by reference to specific examplesbelow. The binder used may be any one which can be put in a liquid orplastic form, as in solution or in the molten state, whereby it willpenetrate and illl all spaces in a mass of aggregates, for

example, a solution of a reslnous binder, or a mixture of a liquidreslnous binder and a nely divided solid reslnous binder, or a moltenthermoplastic binder such as certain resins and metals. Of course thebinder selected should be one which U can be used at a temperature whichwill not injure the binder in the aggregates.

The invention will be described with reference to the use of specificbinders for the grain and for the aggregates. It is to be understoodhow- Il ever that the invention is not limited thereby but includes theuse of any combination of binders to selectively control the cuttingaction oi' the resulting abrasive product. Furthermore, although theinvention specifically provides a n means of using nes from valuableabrasive materials it is evident that the principles of the inventionapply to use of other, larger, or less expensive abrasive materials.

, A type of abrasive article which can be made u in accordance with theapplicants invention is illustrated in the accompanying drawing. Thisdrawing shows a fragmentary section of an abrasive wheel, the sectionbeing taken in a plane at right angles to the axis of the wheel.

"0' Prebonded aggregates are indicated on an enlarged scale, eachaggregate containing a plurality of abrasive particles (such as diamonddust) which have been strongly bonded under heat and pressure. Theaggregates are formed by crushing the bonded mass after it has cooled.The aggregates are thereafter united by means of a dense bond whichmakes intimate contact with the irregular surfaces of the aggregates.

ln the drawing abrasive particles 2 are shown in clusters, each clustercontaining abrasive particles which have been prebonded with a binder 3before the breaking up of the prebonded mass into aggregates. The binder3 may be composed (as mentioned above in some of the examples) Il ofmetal or glass. After the prebonded mass has been broken up intoaggregates the latter are formed into an abrasive article with the aidof an interaggregate binder, such for example as a phenol condensationproduct resin, and the 00 whole article is heated in this case underpressure to form a hard dense bond.

Example iy I 70 diamonds are mixed with metal which is in nely dividedform. The amount of bond material used is that needed substantially tofill the volumebetween the abrasive particles. Simple calculations basedupon the speciiic gravity, appar- 75 ent density, grit size, etc. ofthematerials to be used permit the calculation of this amount withreasonable accuracy. The mixture is pressed and subjected totemperatures ranging from approximately 700 C.'to 1500 C. depending onthe metal used. Diamonds are subject to oxidation 5 at temperaturesabove 700 C. It is desirable .therefore that the heating should beperformed in a non-oxidizing atmosphere such as may be produced bydeposition of ilnely divided carbon on the mix or by the use of anatmosphere of l0 hydrogen. A hydrogen atmosphere facilitates the wettingof the diamond particles by the metal. It is desirable also that theheating should be rapid to minimize the action of any residual oxygen inthe mix on the diamonds. Il The mix can also be subjected to jarringwhile the metal is in a liquid or softened condition in order to bringthe metal into closer contact with the abrasive particles, so that theattractive forces between the metal and the abrasive par- I ticles maybe more fully utilized and so that a dense mass of metal and includedabrasive may be obtained. After the mixture has cooled the mass isbroken up by any convenient method common to the art of crushing intoaggregates il each of which has a plurality of abrasive particles.

Another. hard abrasive that can be thus incorporated with metal is boroncarbide. Boron carbide can be alloyed, or incorporated into intla matemixture, with a number of metals, and metal-bonded aggregates can beprepared by either of the following processes.

Example II u Examples III A mixture of boron carbide and a metal is,heated until the entire mass becomes fluid. On 50 cooling the mass theboron carbide crystallizes to give fairly well developed crystalsembedded in a metal matrix. 'I'he temperature required to producefluidity of the entire mass is usually about 2000 C. or higher,depending on the metal used and on the proportions of metal and boroncarbide. Examples of metals which can be used singly or in combinationin this manner with boron carbide are copper, nickel, cobalt, iron, etc.'Ihe cooled mass is broken up into aggregates containing boron carbidecrystals and interconnecting metal.

In the production of metal bonded aggregates, alloying agents may beincorporated into the metal to produce a satisfactory degree ofbrittleness for crushing purposes and to give the proper grindingaction. 'I'hese addition agents are well known in the metallurgical artand need not be described in detail. As specinc examples, copper can beembrittled by the addition of tungsten, aluminum or tin, the amountadded being dependent upon the properties desiredin the bondedaggregate. Thisalso constitutes a mechanism for controlling the slow butnecessary I breakdown of the metal bonded aggregate duru ing use; thisin turn makes possible considerable control of the cutting properties ofthe abrasive article because the brittleness or' toughness of the bondand abrasive may be adjusted-to suit the character of the cutting actiondesired. Copper may also be embrittled by dissolving therein boroncarbide as disclosed in Example III. Dense masses of abrasiveparticlesand an interconnecting medium can be made by mixing diamondparticles with powdered glass and melting the glass in contact with thediamond particles in a reducing atmosphere. One glass which has beenfound suitable forthis purpose is made by fusing, crushing and screeningthe following ingredients:

' Parts by weight 20 Boi-ax"- i Flint--- 48 Soda ash 12 Zinc oxide 20 Asmall amount of dextrin as a temporary binder may be used whencompacting a mixture of glass and diamonds. For ring, an oxidizingatmosphere is maintained at 600 C. until the dextrin has burned away,then the article may be transferred to a graphite container and quicklyheated for a short time at 900 C. in a reducing atmosphere followed byany convenient cooling schedule.

After a dense mass containing abrasive particles distributed throughouthas been prepared by one of the methods described above, it is broken upinto much smaller aggregates in each of which a number of abrasiveparticles are held in a comparatively non-porous lump or aggregate.These lumps or aggregates are then admixed with a bonding material suchas a heat hardenable resin; e. g. a phenolic condensationv product resinin the A or B stage. In view of the comparatively non-porous characterof the aggregates they can be molded with such a resin bond (and inertfiller if desired) into a compact body having few pore spaces. Thetendency is for the resinous bond to ilow into and illl the pores of theabrasive aggregates. p The molded body is cured at high pressures and attemperatures which produce a curing of the bond, and the body may besubjected to a baking process to further cure the bond.

It is possible to use a wide ra-nge in proportion and variety ofabrasive aggregates, bonds and fillers for making abrasive articles. Onemix which may beused comprises: y

Per cent by weight 40 mesh and finer aggregate clusters of glass bondeddiamonds; the diamonds used comprising a mixtureof grit sizes passing anmesh screen and being retained on a 200 mesh screen 10 Fused aluminaparticles passing a 325 mesh screen 22 B stage phenol-formaldehyde typeresin 12 Previously cured and crushed phenol-formaldehyde type resin,screened to pass a mesh screen 56 and baking the mold and contents for16 hours at 350.F.; upon cooling and dissembly of the mold, the abrasivewheel is ready for use.

Instead of a-bonding material composed of a heat hardenable resin, athermoplastic resin can used. An example of such a resin is apolymerized resin tha-t can be made from a vinyl acetate base. .'Ihevinyl acetate is polymerized by means of light or heat until theviscosity of a molar solution of the resin in benzol is about 15centipoises. About seventy percent of the acetate ygroups are replacedby acetaldehyde. The resultant resin is thermoplastic and is welladapted formolding in combination with fillers to form strong structuresof low porosity. It is also possible to use mixtures of reversible andirreversible types of resin to bring about desired elasticity,

toughness and resilience of the bond.

lent than the glass. In the process of bonding, y the aggregates aresurrounded by molten metal under pressure to make a dense structure.Aside from the aggregates and binders therefor, described above,aggregates may be made using other binders, such as rubber, resinsolutions, and clay.

In making resin bonded aggregates with norma-lly solid heat hardenableresin 220 grit abrasive grain is mixed with a solution of A stage phenolformaldehyde resin in acetone in a proportion to supply 2% of resinbased on the abrasive grain. The mixture is then dried in the" oven for6 to 10 hours at 150 F. and the temperature is then raised to 250 F. andheld at that figure for one half hour. The resulting mass is thencooled, crushed, and screened to produce a fraction of aggregates havingthe desired size.

Rubber bonded aggregates are made from the following ingredients:

, Percent Smoked sheet 8 Sulfur 4 Abrasive grain- 88 ing 5% clay bonawith 95% abrasive grain with suitable additions of liquid to give themixture the proper consistency. The mass is then baked to vitrify thebond at 1250 C. After cooling sui'- ilciently the vitrified mass isbroken up to produce aggregates each containing a plurality of abrasivegrains.

I'he aggregates thus made may be rebonded into dense structures by meansof any binder which is plastic at a temperature which will not injurethe aggregates, to flow into all spaces between the aggregates andspaces communicating `therewith within the aggregates, e. g.thermoplastic resins, low melting metals such as copper and zinc, andmixtures of liquid resin and solid resin.

' Articles made according to this invention may be somewhatsmooth-surfaced when first made. They may be made ready for use bysubjecting the working surfaces to a gentle disking action byoscillating the article slowly over a surface plate charged with iine,granular loose abrasive which cuts away the bond between the aggregatesdue to its relative softness and also small portions of bond from theabrasive'clusters, thus leaving protruding cutting edges. Thlsdiskingprocess, or its equivalent, may be resorted to periodically during theuse of the article to remove any detritus lodged in its working surfaceor to bring new cutting edges into use.

It is an important advantage of the invention that a structure havingrelatively tine abrasive granules may lbe provided with relatively largeclearance spaces between aggregates. Relatively large non-abrasivespaces on the working face oi the abrasive article are illled with thebinder for the aggregates, which spaces may be made concave or hollow byremoval of the binder therefrom by a proper dressing action to provideclearance spaces for the abrasive granules and space for theaccumulation of material removed from the work. If the binder fuorV theaggregates is made relatively softer than the -binder for the grains,the dressing Vaction will tend to remove more material from between theaggregates than in the aggregates, thus providing the proper setting forthe aggregates.

Abrasive wheels made by bonding abrasive aggregates of the typedescribed above and in the manner described have many advantages. Theabrasive particles are surrounded for the most part by material thatadheres more rmly to the abrasive granules than most resinous bonds.Again the abrasive aggregates have a comparatively large size and can bemade with roughened surfaces so that the resinous bond used in unitingthe aggregates can hold the aggregates strongly. Moreover the use ofdiilerent bonding materials in combination with closely compactedabrasive aggregates makes it possible to obtain an exceptionally widerange of cutting characteristics.

'Ihe present invention makes it possible to use more elciently abrasivedusts such as diamondfines, since the aggregates give some of theeffects of larger abrasive particles such as are commonly selected foruse in the manufacture of bonded diamond articles. 'I'here is always anexcess oi' ne grit sized material produced in crushing diamonds toobtain the larger sizes used i'or diamond abrasive wheels. etc.Insuilicient market exists to absorb the quantities of fines producedand the material is therefore rela tively inexpensive. This inventionmakes it possible to utilize the fines -to do a large portion of thework oi coarser, more expensive grits. Similar remarks apply to theutilization oi the ilnerv particles obtained by crushing other rare andcostly abrasive materials.

We claim:

l.l The steps in the method oi making an abrasive article which comprisemixing diamond particles with powdered glass and a carbonaceous binder,heating up the mixture in an oxidizing atmosphere at temperatures belowthe melting point of the glass until the camonaceous binder issubstantially dissipated, then heating the mixture in a reducingatmosphere until the glass is melted around the diamond particles,cooling the mass and crushing the cooled mass to form aggregatesconsisting oi' diamond particles and glass, and bonding the aggregateswith a reactive resin under heat and pressure.

2. The steps in the method of making an abrasive article which comprisemixing diamond particles with powdered boro-silicate' glass and acarbonaceous binder, heating up the mixture in an oxidizing atmosphereat about 600 C. until the carbonaceous binder is substantiallydissipated, then heating the mixture for a short time to about 900 C. tomelt the glass around the diamond particles, crushing the cooled mass toform aggregates consisting of diamond particles and glass, and bondingthe aggregates with a reu active resin under heat and pressure. 3. Anabrasive article comprising aggregates composed of glass-bonded diamondparticles and an interaggregate bond composed oi' synthetic resin curedin intimate contact with said aggregates.

4. An abrasive article comprising aggregates composed of diamondparticles bonded with a borosilicate glass and an interaggregate bondcomposed of a synthetic resin and a hard filler which have beencompressed into intimate con- `dl tact with said aggregates underpressure and heat.

RAYMOND C. BENNER. ALBERT L. BALL.

